South Jersey Skies: So, what's your sign this week?

A reader sent me a plea for understanding last week. She had heard a report that some astronomer had “discovered” a new constellation in the zodiac, and that all the signs had been changed.

“I've always thought of myself as a Sagittarius, and this guy claims I'm not. He says I'm an Ophiuchus. I've never even heard of that sign! What's going on?!”

So today we'll conduct a short workshop on new signs and wonders. For our first lesson, please repeat after me: “Signs are not the same thing as constellations!”

Keith Johnson

At one time, they were more closely related than they are now. Somewhere around 2,500 years ago (the time is a bit uncertain), astrologers looked at the track of the Sun across the stars during the course of a year, called the ecliptic, and decided some organization was required.

First they defined a narrow band, like a man's wedding band on edge, extending a few degrees on either side of the ecliptic, which they called the zodiac.

This band contains all the classical planets. The Sun marches steadily along the ecliptic at the center of the band, and the planets move in their more-interesting ways through the zodiac band, a little above and below the ecliptic.

The seers then divided this band up into 12 equal segments (for whatever reason), starting at the vernal equinox position. These were the signs of the zodiac.

Now they had to name these signs, so they could work with them. They decided to use the names of nearby constellations to label the signs (I admit I'm oversimplifying a great deal here).

View full sizeA part of the sky containing Ophiuchus, the Serpent Bearer, Scorpius, the Scorpion, and Serpens, the Serpent. The thick horizontal line labeled "Ecliptic" is the annual path of the Sun against the background stars. Dates for the Sun's location are indicated. The astronomical boundaries of all the constellations are shown, as are classical drawings for the three main ones. Note that Serpens is the one constellation in our sky that is divided into two parts. Graphic produced with the aid of Starry Night software.

For instance, one part of the zodiac passes south of Leo, the Lion. It doesn't go directly through Leo's body, but Leo is the nearest prominent constellation, so they used that for one of the names.

Astronomers often object to this. For one thing, the signs are all the same length, each spanning 30 degrees along the ecliptic. But the constellations are not nearly the same size. Big ol’ Virgo is three or four times longer than poor little Aries. But even from the beginning, astrologers did not intend the signs to be the same as the constellations.

Astronomers also often object that the two sets of patterns don't even overlay one another now. The Earth's axis slowly changes the direction in which it is pointed, tracing out a cone shape, much like an ice cream cone, over a time of roughly 26,000 years. This motion, called precession, causes the stars defining the constellations to shift westward within the signs, about a degree's worth every 72 years.

By now, over 2,000 years later, the stars have drifted just about one entire sign's worth. If you were born on the first day of spring 2,500 years ago, the Sun would have been in the constellation Aries. Now it's in the middle of Pisces!

But your sign is still Aries. Again, this is nothing new. The signs, for most (“tropical”) astrologers, are defined by the equinoxes and solstices, i.e., by the seasons, rather than by the visible stars.

The coincidence of names is handy for historians, by the way. By looking back in time until the signs and the constellations were roughly in alignment, we can determine approximately when the signs were created, and thus when astrology was invented.

Suppose, however, that you preferred to actually use the stars as your background astrological framework, rather than the ancient signs (there is a minority branch of astrologers, the sidereal ones, who do follow such a preference). Suppose moreover that you were not worried that all your star-signs were different sizes (a very small subgroup takes this additional step). Then you would say your sign was Pisces on March 21, at least currently.

An issue then emerges: how do you define the constellations? The boundaries of even the familiar Western shapes — Orion, Taurus, Ursa Major — have been quite fluid through the centuries. Even the total number of constellations has varied.

Astronomers don't have much scientific use for the ancient stories, but constellations are useful for cataloging and labeling purposes. So, in 1930, the International Astronomical Union — the same folks who demoted Pluto recently — divided the sky up into 88 complex regions like states. Each has straight-line boundaries running north-south and east-west, chosen to enclose the classical pictures as well as possible (the shapes are often a bit bizarre). Anything discovered in the sky could then be assigned by its location to a particular constellation.

If you now trace the ecliptic through this “new” assemblage of official shapes, it passes through 13 constellations, rather than the old 12. The new one is Ophiuchus, the Serpent Bearer, popping up between Scorpius and Sagittarius. That's just a result of how the lines happened to get drawn in 1930.

The Sun spends quite some time there, too: Ophy is a big boy! The Sun scoots through a single claw of Scorpius in less than a week before taking a leisurely 19-day stroll through the Serpent Bearer. (An interesting note: the name of the constellation is “Scorpius,” whereas the name of the sign is “Scorpio.”)

It's not clear why this issue has suddenly attracted the attention of the media. It's been known for a long time. Neither astronomers nor classical astrologers are concerned about it. I've mentioned it to astronomy classes for decades.

But I must admit that, when asked what my sign is, I sometimes tell people, “Ophiuchus.” Actually, I'm an Aquarius, but that's such a boring constellation; and claiming Ophiuchus allows me to embark on a discussion about astrology vs. astronomy, and why I don't believe in all that horoscope nonsense.

We public astronomy educators must grab every opportunity to hook our students on the Real Stuff!

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Keith Johnson is the director of Edelman Planetarium at Rowan University, where the star show “Images of the Infinite: The Hubble Space Telescope” and the laser light show “Laser Doors” is now showing to the general public. Information is available at www.rowan.edu/planetarium/.